This invention relates to a method for directed retry in a mobile communication system. The method consists of the following steps: measuring the reception level of the signals from the neighboring cells of the target cell in the mobile station, sending the measurement results as a report message from the mobile station to the mobile communication network, detecting the need for directed retry, selecting at least one neighboring cell as the candidate cell for directed retry on the basis of the measurements, and executing the directed retry to the selected target cell.
Additionally, the invention relates to a control unit for controlling the directed retry.
FIG. 1 (attached) shows a simplified block diagram of the Pan-European GSM mobile communication system. The MS (Mobile Station) is connected via a radio path to a BTS (Base Transceiver Station), in FIG. 1 to base station BTS1. A BSS (Base Station Sub-system) consists of a BSC (Base Station Controller) and the base stations BTS controlled by it. An MSC (Mobile Services Switching Center) usually controls several base station controllers BSC. The mobile services switching center MSC is connected to other mobile services switching centers and, via a GMSC (Gateway Mobile Services Switching Center), the GSM network is connected to other networks, such as the public switching telephone network PSTN, another mobile communication network PLMN or the ISDN network. The operation of the entire system is controlled by the OMC (Operation and Maintenance Center). The subscriber data of the mobile station MS is permanently stored in the HLR (Home Location Register) of the system and temporarily in the VLR (Visitor Location Register) of the area in which the mobile station MS is currently located.
The mobile station MS continuously measures the signals of the base stations BTS nearest to the cell in which it is located, for example, to determine the base station that offers the best signal and to be ready for a cell handover. Each base station BTS continuously transmits in its cell broadcast channel information intended for all mobile stations MS. This information includes, among other things, the BSIC (Base Station Identity Code), the LAI (Location Area Identifier), and information about the frequencies of the neighboring base stations which the mobile stations MS should be measuring. For example, in the GSM mobile communication system the mobile station MS can measure the reception level and quality of the serving base station and simultaneously the reception level of up to 32 other base stations BTS. When in radio connection with a base station BTS, the mobile station MS frequently sends the measurement results as a report message to the base station controller BSC via the serving base station. The report message contains the signal measurement results for the serving base station and for up to the six best neighboring base stations.
When the mobile station MS moves in the area of the mobile communication network, it normally connects to the base station BTS with the strongest signal. Thus the system attempts to establish any mobile station MS terminated or mobile station MS originated call via this base station BTS. If the base station BTS into which the mobile station MS is connected cannot offer a traffic channel for call setup, for example, because of overload, the system executes a so-called directed retry. In this case, the traffic channel is selected during the call setup phase from a cell other than that in which the signaling was performed. The mobile communication network selects the next strongest base station by using the measurement results delivered by the mobile station MS as a report message during call setup signaling and directs the call to be established to the traffic channel of this base station.
The problem with the directed retry according to the prior art is that the serving base station is selected according to single measurement results, even though the mobile station may just be leaving the coverage of the base station selected. The selection of a base station unsuitable in relation to the movement direction of the mobile station will in this case shortly result in a handover. In other words, the problem is the inability of the directed retry according to the prior art to predict, for how long the selected target cell will be the best cell as far as the mobile station call setup is concerned.
The purpose of this invention is to implement the directed retry in such a manner that, for traffic initiation, the system will select the base station whose signal will probably be sufficient for maintaining the call for longer than any other of the possible base stations.
This will be accomplished by using a system of the type described in the introduction for directed retry for which it is characteristic according to the invention that the measurements performed in the mobile station are used to determine the change in the reception level of the signal of at least one candidate cell, and the target cell for directed retry is selected from the candidate cells by using the determined changes in the reception levels of the signals so that the target cell selected is the candidate cell whose value for the change in the reception level of the signal meets a predefined criterion.
Additionally, the invention relates to a control unit for controlling the directed retry. The control unit is adapted to receive measurement reports, to detect the need for directed retry, to use the results of the measurement report to select at least one neighboring cell as the candidate cell for directed retry, and to initiate the directed retry into the selected target cell. It is characteristic for the control unit, according to this invention, that it is additionally adapted to determine the change in the signal reception level of at least one candidate cell on the basis of the received measurement reports and to select as the target cell for the directed retry by using the determined changes in signal reception levels the candidate cell whose value for the change in the signal reception level meets a predefined criterion.
The invention is based on the idea that the system attempts to determine the direction of movement of the mobile station in relation to the base station of the candidate cell for directed retry by comparing the consecutive measurement results delivered by the mobile station for the base station signal of the candidate cell in question to one another and by monitoring the change in the signal. As the target cell for directed retry is selected the cell for which the change in consecutive measurement results of the base station signal increases most or decreases least. When the base station signal increases rapidly, it may be assumed that the mobile station is moving towards the base station and thus will remain in the coverage of the base station for a longer time than in the coverage of those base stations whose signal strength increases less. If all base station signals decrease, the selected target cell, according to this invention, is one for which the reception level of the base station signal received in the mobile station has decreased least. In one embodiment of the invention it is possible to set a threshold value for the target cell selection, and the change in the base station signal level must exceed this threshold in order for the system to select the traffic channel from the base station in question.
The advantage of this kind of method is that the probability of success for directed retry is improved and thus the probability for the call being cut off decreases.
Another advantage of the method according to this invention is that the need for handovers is decreased.
Yet another advantage in this invention is that the load of the mobile communication network is divided in a more optimal fashion between the different base stations when the call is not always transferred to the same cell with the strongest signal level but to the cell that is best in relation to the movement direction of the mobile station.